[英]Linq and the Equality Operator: Expression of type 'System.Int32' cannot be used for parameter of type 'System.Object'
I'm trying to override the equality (==) operator in C# to handle comparing any type to a custom type (the custom type is really a wrapper/box around null). 我正在尝试覆盖C#中的相等(==)运算符来处理将任何类型与自定义类型进行比较(自定义类型实际上是一个围绕null的包装器/框)。
So I have this: 所以我有这个:
internal sealed class Nothing
{
public override bool Equals(object obj)
{
if (obj == null || obj is Nothing)
return true;
else
return false;
}
public static bool operator ==(object x, Nothing y)
{
if ((x == null || x is Nothing) && (y == null || y is Nothing))
return true;
return false;
}
...
}
Now if I make a call like: 现在,如果我打电话:
Nothing n = new Nothing();
bool equal = (10 == n);
It works perfectly fine. 它工作得很好。 However, if I try to do this same thing through a Linq expression tree: 但是,如果我尝试通过Linq表达式树执行相同的操作:
exp = Expression.Equal(
Expression.Constant(10),
Expression.Constant(new Nothing(), typeof(Nothing))
);
It throws the exception: 它抛出了异常:
System.ArgumentException : Expression of type 'System.Int32' cannot be used for parameter of type 'System.Object' of method 'Boolean op_Equality(System.Object, PARTSFinder.Rules.Runtime.RulesNothing)'
at System.Linq.Expressions.Expression.ValidateArgumentTypes(MethodInfo method, ReadOnlyCollection`1& arguments)
at System.Linq.Expressions.Expression.ValidateCallArgs(Expression instance, MethodInfo method, ReadOnlyCollection`1& arguments)
at System.Linq.Expressions.Expression.Call(Expression instance, MethodInfo method, IEnumerable`1 arguments)
at System.Linq.Expressions.Expression.Call(Expression instance, MethodInfo method, Expression[] arguments)
at System.Linq.Expressions.ExpressionCompiler.GenerateBinaryMethod(ILGenerator gen, BinaryExpression b, StackType ask)
Any ideas on why the base system can convert Int32 to Object, but Linq can't, or how I can fix this? 关于为什么基本系统可以将Int32转换为Object的任何想法,但Linq不能,或者我如何解决这个问题?
This whole thing stared because Linq also can't compare Int32 to Object in the first place: 整个事情盯着看,因为Linq也无法首先将Int32与Object进行比较:
exp = Expression.Equal(
Expression.Constant(10),
Expression.Constant(null)
);
Throws an exception stating that there is no comparison operator for "System.Int32" and "System.Object". 引发异常,指出“System.Int32”和“System.Object”没有比较运算符。
Quick followup: 快速跟进:
The following do work without issue: 以下工作没有问题:
exp = Expression.Equal(
Expression.Constant(10, typeof(object)),
Expression.Constant(new Nothing(), typeof(Nothing))
);
exp = Expression.Equal(
Expression.Constant(10, typeof(object)),
Expression.Constant(null)
);
So specifically casting everything to object. 所以特意将一切都投射到物体上 So does Linq just not handle inheritance internally? 那么Linq内部没有处理继承吗? Thats pretty annoying... 多奇怪啊...
Followup #2: 后续#2:
I also tried using a custom comparison method: 我也尝试过使用自定义比较方法:
exp = Expression.Equal(
Expression.Constant(10),
Expression.Constant(null),
false,
this.GetType().GetMethod("ValueEquals", BindingFlags.Public | BindingFlags.Static)
);
public static bool ValueEquals(object x, object y)
{
if (x == null && y == null)
return true;
if (x.GetType() != y.GetType())
return false;
return x == y;
}
This too throws an exception: 这也引发了一个例外:
System.InvalidOperationException : The operands for operator 'Equal' do not match the parameters of method 'ValueEquals'.
at System.Linq.Expressions.Expression.GetMethodBasedBinaryOperator(ExpressionType binaryType, Expression left, Expression right, MethodInfo method, Boolean liftToNull)
But again casting everything directly to object works: 但是再次将所有内容直接投射到对象上:
exp = Expression.Equal(
Expression.Constant(10, typeof(object)),
Expression.Constant(null, typeof(object)),
false,
this.GetType().GetMethod("ValueEquals", BindingFlags.Public | BindingFlags.Static)
);
So I guess I have my workaround... cast everything to object and use a custom comparison method. 所以我想我有我的解决方法......将所有内容都转换为对象并使用自定义比较方法。 I'm still surprised Linq doesn't do the conversion automatically as normal C# does. 我仍然感到惊讶Linq没有像普通C#那样自动进行转换。
What is wrong with null? null有什么问题? Re the missing int
vs null
, try int?
重新删除int
vs null
,尝试int?
: :
exp = Expression.Equal(
Expression.Constant(10, typeof(int?)),
Expression.Constant(null, typeof(int?))
);
You should take a look at this article to check for a nullable type. 您应该查看本文以检查可以为空的类型。
http://msdn.microsoft.com/en-us/library/ms366789.aspx http://msdn.microsoft.com/en-us/library/ms366789.aspx
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